The present invention generally relates to a receiver architecture for use with spread spectrum and Code Division Multiple Access (CDMA) wireless networks.
Universal Mobile Telecommunications System (UMTS) is a standard for 3G wireless networks, as defined by the International Telecommunications Union (ITU). UMTS defines a communication scheme composed of slots, with 15 slots forming a frame. Each slot specifies, among other data, synchronization information used to synchronize communications between nodes of a UMTS compliant network.
An important procedure performed by a receiver within a UMTS network, for example a CDMA mobile receiver, is the cell search operation. Cell searching typically is performed by a cell search system that is incorporated as part of the receiver. The cell search system is activated after the receiver is powered on to determine synchronization information pertaining to the cell in which the receiver is located. The cell search operation is a three stage process. That is, the cell search system performs slot synchronization (primary synchronization), frame synchronization and scrambling code group determination (secondary synchronization), and scrambling code determination.
In performing cell searching, the cell search system accesses a Synchronization Channel (SCH) and a Common Pilot Channel (CPICH) of the received wireless signal. The SCH is a composite channel formed from a Primary SCH and a Secondary SCH. Within each slot, the Primary SCH specifies a Primary Synchronization Code (PSC). The Primary SCH, however, only contains data during the first 256 chips of each 2560 chip slot. As is known, “chip” or “chip rate” refers to the rate of the spreading code within a CDMA communication system.
The cell search system of the receiver uses the Primary SCH to acquire slot synchronization with a cell. Typically this is performed using a single matched filter, or other similar device. The filter is matched to the PSC which is common to all cells. The slot timing of a cell can be obtained by detecting peaks in the matched filter output.
The cell search system of the receiver performs frame synchronization using the Secondary SCH. The Secondary SCH specifies, within each slot, a Secondary Synchronization Code (SSC). Unlike the PSC, the SSC can be one of 16 different codes. Each slot contains one SSC. The SSC used varies from slot to slot to form a sequence that has a period of one frame, or 15 slots. There are 64 possible SSC sequences and each sequence corresponds to one of 64 possible scrambling code groups. By observing a full frame of data, the receiver can determine which of the 64 SSC sequences is being transmitted. Since the SSC sequence repeats with a period equal to one frame, the sequence can be used to achieve frame synchronization at the receiver because frame boundaries can be identified. The SSC sequence that is transmitted further indicates which scrambling code group is used in the current cell.
Each scrambling code group includes 8 possible scrambling codes. To determine the actual scrambling code, the cell search system of the receiver correlates the received CPICH signal with each of the 8 possible scrambling codes in the identified scrambling code group until the correct scrambling code is determined. After the actual scrambling code has been identified, the Primary Common Control Channel (CCPCH) can be detected so that system and cell specific Broadcast Channel (BCH) information can be read.
A conventional cell search system typically includes a matched filter correlator for correlating the received samples against the CPICH sequence. Peak detection hardware also is included to locate the peak of the 8 correlations for a given scrambling code group. Other logic also is included to determine which of the peaks is the strongest of the 8 so that a particular scrambling code from the 8 in the scrambling code group can be identified.
Conventional cell search designs are expensive both in terms of the amount of hardware required and the increased power usage. Traditionally, a cell search system is implemented as an application specific integrated circuit (ASIC). Because the system is large, an increased die size is required for fabrication. Larger ASIC designs include more gates, which consume more power. This can be critical with respect to battery life in a mobile receiver. These problems are exacerbated in that scrambling code determination is performed infrequently—typically only when the receiver is powered on or loses lock.
As such, it would be desirable to reduce the amount of hardware required within a receiver for the cell search process and thus reduce the power consumption of the cell search system in the receiver.